Cutaneous primary afferent neurons are complicated cells with specialized sites which include receptors responsible for receiving stimuli and transducing them into electrical potentials, axons capable of conducting to the spinal cord impulses generated in the periphery, and central terminals capable of releasing transmitter onto secondary neurons. The functions of these neurons can be regulated at these sites by various chemical, ionic, or synaptic means. The overall objectives of the present investigations are to understand more fully the mechanisms responsible for regulating the activity of afferent neurons. These aims follow from results of my recent previous studies which have centered on the control mechanisms affecting afferent terminals; such studies are not only widened, but will now be extended to include the peripheral end of the same cells. The project will continue investigations of the amphibian sensory system in the spinal cord maintained in vitro and will utilize electrophysiological and pharmacological techniques to study different parts of the afferent neuron in isolated preparations. The proposed experiments seek to elucidate the roles various chemicals located in the skin and nerves play in the excitation of nociceptive cutaneous receptors and/or in their sensitization to noxious stimuli. Investigations of this problem will use a skin-nerve preparation to determine the changes in single cutaneous afferents produced by noxious stimuli and by "algesic" substances. Ion-sensitive microelectrodes inserted in the gray matter of the spinal cord will be used to determine what neuronal elements and/or transmitters are responsible for the rises in extracellular K+ concentration produced by afferent activity and presumably important in regulating the excitability of afferent terminals. Intracellular recording and voltage clamping of afferent cell bodies will be used to ascertain the mechanism of action of catecholamines on the membranes of afferent neurons. The production of pain is a major medical, and human, concern. Thus, the proposed investigations pertain not only to an important area of neurobiology, but are also relevant to understanding the mechanisms of a symptom central to many disease states.